private function incrementMaxPointsBySkill(ilAssQuestionSkillAssignment $assignment)
 {
     $key = $this->buildSkillKey($assignment->getSkillBaseId(), $assignment->getSkillTrefId());
     if (!isset($this->maxPointsBySkill[$key])) {
         $this->maxPointsBySkill[$key] = 0;
     }
     $this->maxPointsBySkill[$key] += $assignment->getMaxSkillPoints();
 }